Linear polyethylenes in general, and high density polyethylenes in particular, and high molecular weight high density polyethylenes more particularly, have gained wide use, for instance, as grocery (shoppers) retail sacks (also known as tee-shirt bags) and extrusion blow molded bottles or containers. However one of the problems that has continued to be bothersome in most of these applications is the linear high density polyethylene material's tendency to be "splitty". This splittiness is generally thought to be due to the fact that a linear high density polyethylene has very few branches, certainly few, if any, long chain branches off of the polymer back bone. Such branching might permit entanglement which could, in certain instances, prevent a tear from being propagated once initiated, and/or retard the growth of a tear. Accordingly, once a tear or puncture is initiated in such substantially linear polyethylenes, if there is a load on the polyethylene film, a bag made from the film, or a container it will tend to "zipper" or split at a rapid rate causing failure of the bag and possible evacuation and damage of the contents.
Many approaches to solving this zippering or splittiness problem have been attempted. Usually the inclusion of an amount, for instance 5 to 10 weight percent of linear low density polyethylene (density in the range of from about 0.915 to about 0.930 g/cm.sup.3) has been moderately successful perhaps due to the branching provided by the LLDPE(more branching in general is observed for LLDPE than HDPE). The linear low density polyethylene (LLDPE) while providing some improvement in dart drop impact and puncture propagation tear also causes unacceptable stretching or elongation at or near the heat seal under an applied load. Such stretching can cause a bag, for instance a heavily laden grocery bag, to deform generally rendering the bag unacceptable, or at least less functional for its intended purpose.
Conventional low density polyethylene (LDPE) has been blended with HMW HDPE and also gives some relief from the splitting or zippering problem, but again, bags made from films based on such a blend are generally unacceptably stretchy as well.
However, while both of these solutions to the splitting problem are used in the industry, both of these solutions present an inherent difficulty. That difficulty is that the HMW-HDPE is relatively high in melt viscosity while in general, both LDPE and LLDPE are relatively much lower in melt viscosity. This leads to inhomogeneity in the melt and in the resulting film, leading to areas of good performance and areas of poor performance, which is a similarly unacceptable solution to the zippering problem. Attempts to homogenize such a mixture to improve the dispersion, thereby improving the overall physical properties have generally met with poor success as well, because while getting the lower viscosity materials more uniformly dispersed, the higher molecular weight or higher viscosity materials (HMW-HDPE) tend to be subjected to heat and sheer causing some cross-linking and thereby a generally unacceptable diminution of bulk film properties.
Ways to gauge the improvement or lack of improvement in tear properties include standard tests in the film industry such as tear or Elmendorf tear, where generally a nick or cut is placed in the film to be tested. While these tests are of value in HDPE film testing, a more realistic commercial indicator of film properties, especially for HMW-HDPE, is the puncture propagation tear (PPT) and the puncture propagation tear length (PPTL). These two tests are indicative of a tear that would be similar to one initiated in an everyday situation by the puncture of a film, or a bag based on the film, and the subsequent length of the tear for a specific puncture.
Another way to improve the tear and puncture propagation tear properties of an HMW-HDPE material might be to extrude, adjacent to the HDPE, for instance in a coextrusion, a more highly branched polymer material. This, however, is a more expensive method of solving this problem and is therefore generally commercially unacceptable.
Also, in the category of solutions to the tear problem, are certain machine or extruder manipulations that can improve the tear strength. Because of the very high melt strength of HMW-HDPE, the film is generally fabricated using a high stalk configuration. This configuration allows for the melt to relax prior to blowing up to the full lay-flat width desired for the finished product. To achieve more relaxation which in turn improves tear and impact properties, film fabricators attempt to maximize the stalk height and the blow-up ratio. Typically the stalk height is six to eight times the die diameter and the blow-up ratio ranges from three to five times the die diameter. These parameters are limited by the stability of the bubble at reasonable production rates.
U.S. Pat. No. 5,110,685 discloses blends of high density polyethylene with elastomers to produce low friction, abrasion resistant coatings. Specifically this document discloses a multi part blend:
a) a high density PE blend of PA1 b) an EPDM elastomer. PA1 Dart Drop Impact: ASTM D-1709 PA1 Tensile Strength: ASTM D-882 PA1 Elmendorf Tear (MD and TD): ASTM D 1922 PA1 Elongation: ASTM D 882 PA1 Puncture Propagation Tear: ASTM D 2582-93 PA1 PP Tear Length (mm) MD and TD: ASTM D-2582-93 PA1 Melt flow rate (ASTM D-1238) @ 200.degree. C., 15 Kg=20 to above 0.1, preferred 12 to above 0.1 dg/min., more preferred 8 to above 0.1 dg/min. PA1 Molecular weight (weight average molecular weight) 20,000 to 100,000 preferred 40,000 to 100,000, more preferred 80,000 to 100,000. Styrene content in the range of from about 10 to 50 percent, preferably, in the range of from about 20 to about 40 percent, more preferably in the range of from about 25 to about 35 percent. EPR, EPDM and butyl rubber are well known polymers and are widely available from a number of commercial sources.
i) high molecular weight HDPE present from about 10 to 80 percent by weight; PA2 ii) medium molecular weight HDPE present from about 20 to 70 percent by weight; and PA2 iii) low molecular weight HDPE making up the balance of the HDPE blend;
Various blends of a) and b) with carbon black and optionally talc, are tested to determine their abrasion resistance and, as a measure of low friction, the coefficient of friction. Such a blend of different MW HDPEs would yield a film that would generally be unsuitable for consumer bags because of dilution of the MW of HMW-HDPE products generally leads to reduced impact properties. This phenomenon is well known to those of ordinary skill in the art regarding performance of medium MW products relative to performance of HMW products. Dart impact (another important physical property of films) of medium MW properties are typically 50 % less than those achieved from HMW based products.
There is therefore a need to produce a high molecular weight high density polyethylene film that can be fabricated into, for instance, tee-shin bags, that have superior resistance to puncture propagation tear and improved puncture propagation tear length and are generally commercially practicable from an economic and fabrication standpoint.